Epidemiology Practice Questions

Q: In a study, 100 patients were telephoned and asked whether they exercise daily (exposure), and whether they have coronary artery disease now or ever before (outcome). Identify the study design.

Cross-sectional study. ***Cross-sectional study*** - This design measures **exposure** and **outcome** simultaneously at a single point in time, commonly referred to as a **prevalence study** or 'snapshot' study. - Because it assesses status at one moment, it cannot determine **temporality** or causality, making it ideal for determining disease prevalence rather than incidence. *Cohort study* - In a cohort study, researchers start with **exposure** status and follow participants over time to observe the development of an **outcome**. - It assesses incidence and is longitudinal, requiring a **follow-up period** which is absent in this scenario. *Case–control study* - This design involves identifying participants based on the presence of the **outcome** (cases) versus those without it (controls) and then looking back for past **exposures**. - Since the study described begins by asking about exercise habits without grouping participants by disease status first, it is not a retrospective case-control study. *Randomized controlled trial (RCT)* - An RCT is an **experimental study** where the investigator **actively intervenes** by randomly assigning patients to treatment or control groups. - This scenario is an **observational study** where the researcher only observes existing habits, rather than assigning exercise regimens.

Q: In a contraceptive study, 500 women were followed for 6 months. During this period, 4 pregnancies occurred. What is the Pearl Index?

1.6. ***1.6*** - The **Pearl Index** is calculated as (Number of accidental pregnancies × 1200) / Total months of exposure. - With 4 pregnancies over 3000 woman-months (500 women × 6 months), the calculation is (4 × 1200) / 3000 = 4800 / 3000 = **1.6**. *2.5* - This value is mathematically incorrect based on the standard formula for the **Pearl Index**. - It likely represents a calculation error where the numerator or denominator was inverted or miscalculated. *5.0* - This result would be reached if one incorrectly multiplied the pregnancies by 1200 and divided by 500 women without accounting for the **months of exposure**. - It fails to utilize the total **person-time of observation**, which is crucial for determining contraceptive failure rates. *2.0* - This would result from an incorrect assumption, such as calculating based on a shorter duration or misinterpreting the **total woman-months**. - It does not adhere to the validated epidemiological formula used to assess **contraceptive efficacy**.

Q: A health centre conducts screening of women in the community. An elderly woman is detected to have carcinoma of the cervix/breast during this screening. This activity corresponds to which level of prevention?

Secondary. ***Secondary*** - **Secondary prevention** involves **early detection** and **prompt treatment** of disease to stop its progression. - **Screening programs** like Pap smears for cervical cancer or mammography for breast cancer are classic examples of this level. *Primary* - **Primary prevention** includes measures applied *before* the disease occurs, such as **vaccination** or health education. - Its goal is to prevent the **onset of disease** rather than detecting an existing one. *Primordial* - **Primordial prevention** targets the underlying environmental and social conditions that lead to the causation of disease (e.g., **policies** promoting healthy diet or banning smoking). - It aims to prevent the emergence of **risk factor patterns** in the population rather than individual screening. *Tertiary* - **Tertiary prevention** applies when the disease is already established and symptomatic. - It focuses on **rehabilitation** and preventing **disability** or complications from the clinical disease.

Q: On investigating an outbreak, an epidemiologist plots epidemic curve of acute febrile illness showing successive clusters separated by approximately 1 incubation period apart. Which of the following diseases shows this pattern of propagated spread?

Measles. ***Measles*** - This infectious disease typically exhibits a **propagated epidemic curve** because it is transmitted directly from person-to-person through respiratory droplets. - Successive waves of cases appear at intervals corresponding to the **incubation period**, as susceptible individuals are infected by the initial index cases, leading to secondary and tertiary generations of transmission. *Salmonellosis* - Usually presents as a **point-source epidemic**, where multiple individuals are exposed simultaneously to a single contaminated food or water source. - The epidemic curve typically shows a **rapid rise and fall** in cases rather than the progressive waves characteristic of person-to-person transmission. *Typhoid* - Like other foodborne or waterborne illnesses, it frequently follows a **common-source or continuous-source pattern** of exposure rather than serial transmission chains. - While secondary spread is possible in poor sanitation settings, the classic presentation is a **sharp peak** resulting from ingestion of contaminated materials. *Food poisoning* - Characteristically results from a **common-source outbreak**, where all affected individuals consume a shared, contaminated food item within a short window. - These cases typically cluster together within a timeframe defined by the **incubation period** after a single meal, rather than showing extended, wave-like generation periods.

Q: In a defined population, approximately 67% of all stroke cases are attributable to uncontrolled hypertension. If hypertension were adequately managed, 2/3rd of stroke burden could be eliminated. This is an example of:

Population attributable risk. ***Population attributable risk*** - This measure represents the **proportion of disease in the total population** that can be attributed to a specific exposure, which is directly applicable to the scenario where 67% of stroke cases are linked to hypertension. - It estimates the **potential impact** of a public health intervention, such as eliminating hypertension, on the total disease burden in the population. *Relative risk* - This is a ratio of the **incidence in the exposed group** compared to the incidence in the unexposed group. - It measures the **strength of the association** between a risk factor and an outcome, rather than the proportion of the disease attributable to the risk factor. *Attributable risk* - Often called **Risk Difference**, this measure calculates the actual difference in incidence rates between exposed and unexposed groups. - It does not account for the **prevalence of the exposure** in the entire population, unlike population attributable risk which considers the total disease burden. *Odds Ratio* - This statistical measure determines the odds of an exposure occurring in cases compared to the odds of exposure in controls, commonly used in **case-control studies**. - It is an estimate of relative risk (when disease is rare) but does not quantify the **portion of population disease** caused by the risk factor.

Q: Health data from 15 nations investigates the relationship between per capita tobacco consumption and national COPD prevalence, revealing a strong positive correlation. Which study type best describes this?

Ecological study. ***Ecological study*** - An ecological study assesses the relationship between exposure and outcome at the **population level** rather than the individual level, using data aggregated across nations as in this example. - The unit of analysis is the **entire population** (15 nations), making it the correct classification for this type of correlational research. *Case control study* - Requires individual-level data by identifying subjects based on the presence or absence of the **outcome** (COPD) and looking back for the **exposure** (tobacco). - It is not used to analyze aggregate national-level data like per capita consumption metrics. *Cohort study* - Involves following a group of individuals over time to determine if exposure leads to the subsequent development of the outcome. - Because this study assesses prevalence and consumption across nations simultaneously at the population level, it does not involve the prospective follow-up characteristic of a cohort study. *Cross sectional study* - Analyzes individual-level data to capture a snapshot of exposure and disease at a single point in time. - Because this study uses group-level (nations) data rather than individual subject data, it is specifically referred to as an ecological study, not a cross-sectional study.

Question 1

Define net reproduction rate

Accepted Answer

Total number of girl children born during the reproductive age of a woman taking into account mortality/Average no of female children a newborn girl will bear during her reproductive age assuming age specific fertility and mortality rates

Answer

Total number of children born during the reproductive age of a woman

Answer

Total number of girl children born during the reproductive age of a woman

Answer

Average number of live births per woman during her reproductive lifespan

Question 2

In a study, 100 patients were telephoned and asked whether they exercise daily (exposure), and whether they have coronary artery disease now or ever before (outcome). Identify the study design.

Accepted Answer

Cross-sectional study

Answer

Cohort study

Answer

Case–control study

Answer

Randomized controlled trial (RCT)

Question 3

In a contraceptive study, 500 women were followed for 6 months. During this period, 4 pregnancies occurred. What is the Pearl Index?

Accepted Answer

1.6

Answer

2.5

Answer

5.0

Answer

2.0

Question 4

A health centre conducts screening of women in the community. An elderly woman is detected to have carcinoma of the cervix/breast during this screening. This activity corresponds to which level of prevention?

Accepted Answer

Secondary

Answer

Primary

Answer

Primordial

Answer

Tertiary

Question 5

On evaluating an influenza vaccine, it demonstrated an efficacy of 95% in randomized controlled
trials and an effectiveness of 90% in post-marketing surveillance studies. Which of the following
statement is incorrect?

Accepted Answer

The vaccine confers absolute and complete protection against influenza infection

Answer

Vaccinated individuals demonstrate a 95% reduction in disease risk vs unvaccinated

Answer

Vaccinated individuals show a 90% reduction in influenza risk in real-world settings

Answer

Vaccine effectiveness can be lower than efficacy in real-world settings

Question 6

On investigating an outbreak, an epidemiologist plots epidemic curve of acute febrile illness
showing successive clusters separated by approximately 1 incubation period apart. Which of the
following diseases shows this pattern of propagated spread?

Accepted Answer

Measles

Answer

Salmonellosis

Answer

Typhoid

Answer

Food poisoning

Question 7

In a defined population, approximately 67% of all stroke cases are attributable to uncontrolled
hypertension. If hypertension were adequately managed, 2/3rd of stroke burden could be
eliminated. This is an example of:

Accepted Answer

Population attributable risk

Answer

Relative risk

Answer

Attributable risk

Answer

Odds Ratio

Question 8

Health data from 15 nations investigates the relationship between per capita tobacco
consumption and national COPD prevalence, revealing a strong positive correlation. Which study
type best describes this?

Accepted Answer

Ecological study

Answer

Case control study

Answer

Cohort study

Answer

Cross sectional study

Question 9

Assertion: In a nested case-control study, incidence rates can be derived from the parent cohort.
Reason: Nested case-control studies have the ability to estimate relative risk accurately.

Accepted Answer

Both assertion and reason are correct, but they are unrelated

Answer

Both assertion and reason are correct, and the reason is the correct explanation of the assertion

Answer

Assertion is correct, but the reason is incorrect

Answer

Assertion is incorrect, but the reason is correct

Question 10

Following a community feast in a rural village in Rajasthan, the district health officer receives reports of 47 individuals presenting with acute watery diarrhoea, nausea, and vomiting within a 72-hour window. The officer plots the onset dates of all confirmed cases. The epidemic curve (Figure 1) shows a sharp single peak on day 3 followed by rapid decline, with no secondary waves. Based on Figure 1, which of the following conclusions is most appropriate to guide the investigation?

Accepted Answer

B. The outbreak pattern is consistent with a common-source, single-exposure event; the implicated exposure likely occurred within one incubation period before the peak

Answer

A. The outbreak pattern suggests a propagated source; contact tracing of secondary cases should be prioritised

Answer

C. The outbreak pattern indicates an environmental continuous-source exposure; water supply chlorination should be initiated immediately without further food history

Answer

D. The bimodal distribution of cases indicates two separate exposure events; both should be investigated independently

Epidemiology — MCQs

Epidemiology — MCQs

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